Balanced hydraulic device

ABSTRACT

In an axial piston hydraulic device, having an angularly tiltable tilt plate for controlling the stroke of the pistons, the cylinders in the rotor communicate with the high and low pressure ports of the device by way of passages which are positioned between the axes of the cylinders and the rotational axis of the rotor to reduce resistances to fluid flow resulting from rotor rotation when the cylinders are on their low pressure stroke. Additional passages are provided which communicate the high pressure port with the rotor and/or the bearing of the tilt plate to exert counterbalancing forces on these components during operation of the device. On- and off-stroke control pistons may also be positioned on the side of the tilt plate opposite the pistons to exert counterbalancing forces on the tilt plate.

United States Patent Ankeny et a1.

[ 1 Aug. 8, 1972 [54] BALANCED HYDRAULIC DEVICE [72] Inventors: Jay H.Ankeny, West Des Moines;

Harold W. Foddy, Woodward; Dayton F. Kessler, Des Moines, all of 21m 7[73] Assignee: Delavan Manufacturing Company, Inc. A [22] Filed: March31,1970

[21] Appl. No.: 24,163

[52] US. Cl ..91/487, 91/506 [51] Int. Cl. ..F0lb 13/04 [58] Field ofSearch ..417/203, 205, 206; 91/499, 91/484-489, 565, 507

[56] References Cited UNITED STATES PATENTS 3,126,835 3/1964 Kline..91/507 2,967,491 1/1961 Wiggermann ..91/505 2,155,455 4/1939 Thoma..91/489 3,194,172 7/ 1965 Schottler ..91/485 3,171,361 3/1965 Boulet..91/505 Davis, Jr. et .91 I507 FOREIGN PATENTS OR APPLICATIONS1,812,801 12/1969 Germany ..91/499 791,992 3/1958 Great Britain ..91/499Primary Examiner-William L. Freeh Attorney-Molinare, Allegretti, Newitt& Witcoff [57] ABSTRACT In an axial piston hydraulic device, having anangularly tiltable tilt plate for controlling the stroke of the pistons,the cylinders in the rotor communicate with the high and low pressureports of the device by way of passages which are positioned between theaxes of the cylinders and the rotational axis of the rotor to reduceresistances to fluid flow resulting from rotor rotation when thecylinders are on their low pressure stroke. Additional passages areprovided which communicate the high pressure port with the rotor and/orthe bearing of the tilt plate to exert counterbalancing forces on thesecomponents during operation of the device. Onand off-stroke controlpistons may also be positioned on the side of the tilt plate oppositethe pistons to exert counterbalancing forces on the tilt plate.

INVENTORS uAY H. ANKE/Vn BY HAROLD 14 F000) 9 DAYTON KESSLER ATTORNE YSsum 1 or 4 PATENTEl'lAus' 8 I972 BALANCED HYDRAULIC DEVICE BACKGROUNDAND SUMMARY OF THE INVENTION This invention relates to a pressure deviceand, more 5 particularly, to a hydraulicallybalanced axial piston pumpor motor.

Where a hydraulic pressure pumping device is tobe supplied with liquidfrom a source at atmospheric pressure, starvation of the cylinders ofthe pump which are on the suction stroke frequently occurs as a resultof resistances developed within the passages supplying fluid to thecylinders which oppose the filling of the cylinders. These resistancesarise as the result of several design features common to such axialpiston pumps. One of the design features of such pump which contributesin more than an insignificant amount to the creation of resistance, isthe necessary radial spacing of the pumps cylinders and the passageswhich supply the cylinders from the rotational axis of the rotor of thepump. This spacing, combined with the relatively high rotational speedsof the rotor, imparts centrifugal as well as tangential forces to thefluid enroute to the cylinders. These forces, as well as axial forcesdue to orifice effect, tend to resist the flow of fluid into thecylinders. Where the pump is large and where the fluid is to be suppliedat atmospheric pressure, these resistances may become sufficiently largeso as to preclude the use of the pump without appropriate auxiliarybooster devices. In addition, where the pump is operated at or near suchstarvation levels, both an increase in wear and operational noise levelresult.

In one principal aspect of the invention, the pump constructed inaccordance with the principles of the invention, is capable of asubstantial reduction of the likelihood of starvation and the attendantundesirable consequences of starvation or near starvation. In the pumpof the invention, resistance resulting from centrifugal as well astangential effect may be substantially reduced. By reducing theseresistances, the pump constructed in accordance with the principles ofthe invention may be larger than those previously employed for pumpingfluid from fluid sources at atmospheric pressure and the need forbooster devices is substantially reduced. In the pump of the invention,the passages which communicate the ports with the cylinders of thedevice are radially offset from the cylinders of the pump toward therotational axis of the rotor so as to substantially reduce resistance tothe flow of fluid into the cylinders during the suction stroke of thepistons.

Also in such axial piston devices, substantial unbalancing forces andmechanical couples are encountered during operation on variouscomponents of the device including the tilt plate, piston shoes androtor. These unbalancing forces not only increase the wear on thesecomponents, but also result in substantial increases in the noise levelof the pump-during operation.

In the fluid pressure device constructed in accordance with theprinciples of the invention, these several unbalancing forces andcouples are counterbalanced to effect a substantial reduction in bothwear and operational noise of the device. Counterbalancing isaccomplished by one or both the direction of high pressure fluid tovarious of the components and the positioning of the tilt plate controlpistons. This high pressure fluid may be supplied directly from the highpressure port of the device.

In one preferred embodiment of device, fluid at high pressure from thehigh pressure port of the device is ported, by way of passages, both tothe cylinders and to the end of the rotor adjacent the high pressureports and there exerts forces on the rotor along its rotational axis andadjacent the cylinders of the device which communicate with the highpressure port respectively so as to offset or counterbalance coupleswhich tend to tilt the rotor of the device.

In another preferred embodiment of device, high pressure fluid from thehigh pressure port of the device is ported to the tilt plate of thedevice to exert a force on the tilt plate which tends to counterbalancethe oppositelydirected forces exerted by the working pistons duringoperation of the device. This fluid force is preferably directed to thebearing of the tilt plate so as to form a fluid cushion upon which thetilt plate is pivoted. This cushion not only results in reduced bearingwear, but also sound insulates the device against the transmission ofnoise through the device during operation. In addition, excess pressureobtaining in the high pressure port of the device may be relievedthrough the latter mentioned bearing of the invention.

In addition to directing high pressure fluid so as to counterbalance theforces generated in the device during normal operation, the controlpistons, which selectively vary the angular disposition of the tiltplate, are also positioned on the side of the tilt plate opposite theworking pistons of the device. Thereby, the forces exerted by thesepistons during operation, also tend to counterbalance the forces exertedon the tilt plate by the working pistons of the'device, as well aseflect a substantial reduction of the overall diameter of the device.These controlpistons may also be operated by way of high pressure fluidported thereto from the high pressure ports of the device.

In a fluid pressurev device incorporating the principles of theinvention, sound transmission during operation is substantially reducedby way of fluid insulation.

These and other objects, features'and advantages of the presentinvention will be more clearly understood from a consideration of thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS In the course of this description,reference will frequently bemade to the attached drawings in which:

FIG. 1 is a cross sectioned plan view of a hydraulic pump incorporatingthe principles of the invention, the right end port cover having beenrotated for the purpose of clarity;

FIG. 2 is an end elevation view of the pump as viewed from the right inFIG. 1;

FIG. 3 is a cross sectioned elevation view of the port plate of the pumptaken substantially along line 3 3 of FIG..1;

FIG. 4 is a cross sectioned elevation view showing the other side of theport plate and takensubstantially along line4-40fFIG. 1;

FIG. 5 is a cross sectioned elevation view of the rotor takensubstantially along line 5 5 of FIG. 1;

FIG. 6 is a schematic showing centrifugal, axial and tangential effectson the fluid entering the device during sucu'on;

FIG. 6A is a plot of pressure v. passage radius from the centerline ofthe rotor and showing the effect on of the hydrostatic pressure passagesof FIG. 7;

FIG. 9 is an exploded view of the tilt plate trunnion bearing; and

FIG. 9A is a view of the opposite side of the trunnion bearing insertshown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the pumpincorporating the principles of the invention is of the axial pistonhydraulic type in which a tilt plate is provided for determining and/oradjusting the stroke of the pistons and hence capacity of the pump.

By way of general description, the pump includes a pair of end covers 10and 11 having a cylindrical housing 12 extending therebetween. The endcovers and housing are drawn firmly together by a plurality of tie bolts14 extending parallel to the axisof the housing between flanged ears 16on the covers and suitable seals 17 are provided to form a sealedenclosure.

Referring to FIGS. 1 and 2, end cover 10 includes a pair of relativelylarge circular ports 16 and 18, port 16 being a section port and theother port 18 a discharge port. Each of these ports is adapted to becoupled to suitable hydraulic lines (not shown) for supplying the pumpwith hydraulic fluid and for receiving the discharge of the pump.

As shown in FIGS. 1-4, a port plate 19 is stationarily attached to theinner face 20 of the cover 10 by way of one or more pins 22. The portplate 19 has a plurality of ports 26, 27, 28 and 29 for communicatingports 16 and 18 respectively with the respective ones of the cylindersof the pump.

' A rotor 30 is next positioned in the housing such that its end face 32rotatably bears against the left face 34 of the port plate as shown inFIG. 1. The rotor is spaced from the inner wall of the cylindricalhousing 12 and is rotatable relative thereto. A hydrodynamic journalbearing 35 is positioned between the rotor and cylindrical housing toreceive the lateral load on the rotor as a result of the pistonsconjunction with the tilt plate. Passages 35' extend angilarly throughthe rotor to provide a pumping action between the sides of the rotor.This pumping action provides a source of cool fluid from the casing tothe bearing.

The rotor is driven by way of a drive shaft 36 which extends through endcover 11 where it is adapted to be coupled with a suitable drive source(not shown). The drive shaft 36 also extends through the-tilt plate andits bearing and the inner end of the shaft is formed with a plurality oflongitudinally extending external splines 38 which are adapted to engagecorresponding internal splines 39 in a recessed portion 40 of the rotor,whereby the drive shaft rotatably drives the rotor. One or more dishedspring discs 42 are provided between the end of the shaft 36 and therotor to allow for some degree of axial play between the rotor and thedrive shaft during operation, yet urge the rotor into rotating contactwith face 34 of the port plate 19.

The rotor 30 also includes a plurality of cylinders 44 5 which arespaced parallel from each other and also 10 extends outward beyond thesleeve and toward the tilt plate.

The tilt plate assembly comprises an annular plate which is tiltableupon a trunnion bearing assembly, generally 52, so as to vary the strokeof the pistons 48. The face 54 of the tilt plate toward the rotorprovides a flat cam surface upon which a plurality of piston shoes 56are positioned, the bases 57 of each of the piston shoes being slidablerelative to the face 54 of the tilt plate. Each of the piston shoes 56also includes a socket portion 58 which extends outward from the base 57through apertures 60 in a piston shoe holddown plate 62. The sockets 58receive a complementary ball 64 formed integrally on the end of therespective pistons 25 48 and provide for pivotal movement of the pistonsrelative to their shoes. The holddown plate 62 is clamped in overlyingrelationship to the bases 57 of the piston shoes by suitable brackets 66attached about the periphery of the tilt plate as by bolts 68. The tiltplate 50, holddown plate 62, and piston shoes 56 are of well knowndesign and will not be described further in detail. In addition,according to preferred conventional construction, a longitudinal oilpassage 70 extends through the length of the pistons 48 to provide forlubrication of the ball sockets and the sliding surfaces of the bases 57of the piston shoes 56 by way of passage 71 and also for hydrostaticbalance of the balland bases of the pistons and shoes.

In order to adjustably vary the pump capacity, one or more controlpistons are also provided for varying the tilt of the tilt plate 50. Asshown in FIG. 1, piston 72 is an off-stroke piston and piston 74 is anon-stroke piston. In addition to pistons 72 and 74, a spring loaded 45piston 76 may also be provided which exerts a continu- 55 cylinders fromthe rotor axis. These passages 78 and 80 are formed by passages 82 inthe rotor which communicate with cylinders 44 at their innermost side.In turn, passages 82 communicate in sequence with ports 27, 26, 28 and29 in port plate 19 as the rotor rotates.

60 These ports are also spaced closer to the rotor axis than thecylinder axes. Ports 26 and 27 in port plate 19 both communicate by wayof a varied cross section passage 84 with suction port 16 and ports 28and 29 commu- 5 nicate with port 18 also by way of a varied crosssection passage 86. Passages 84 and 86 where they open to the port plateports, are of substantially the same cross section as the latter ports.

. The offset of passages 82 toward the rotor centerline or axis makes itpossible to utilize larger pumps where the pump is to be supplied with asource of fluid which is at atmospheric pressure and obviates the needfor a booster or priming pump to prevent starvation of the pump duringthe suction stroke. Where the pump size is to be increased with anattendant increase in the radial spacing of the respective axes of thecylinders and the passages 82 from the rotation axis, both thetangential and centrifugal effect due to rotor rotation is such thatresistance to filling of the cylinders on the suction stroke increases.

Referring to FIG. 6, the pressure which must be present to fill thecylinders without starvation occurring must be sufficient to overcomethe resistance to filling due to three separate factors, the axialeffect, the tangential effect and the centrifugal effect.

The axial effect creates a pressure resistance due to the pressure dropencountered in passage 82 at its ported interface adjacent the portplate 19. This effect, denoted P varies inversely to the square of thesmallest cross sectional area of the passages 82. This axial resistancemay be represented by the formula where P,, is the axial resistancepressure, Q is the flow rate and A.is the minimum cross sectional areaof the passages 82.

The tangential effect also creates a resistance to filling by way of thetendency of the inflowing fluid to miss the rotor passage 82 as itspeeds by. This effect may be likened somewhat to the problemencountered in boarding a merry-go-round as it speeds by. The tangentialeffect is denoted by P in FIG. 6 and varies by the formula where RPM isthe rotational speed of the rotor, and r,, is the maximum radial spacingof the passages 82 from the rotational axis a of the rotor.

The pressure in passage 84 must be at least of a mag, nitude which issufficient to generate a resultant force or pressure P which willovercome both these effects or the pump will be starved for fluid. Inaddition to the resistances P and P to filling the cylinders, aresistance also results from a centrifugal effect due to the rotationalspeed of the passages 82. This resistance may be generally representedby the formula where P is the centrifugal effect, RPM and r representthe variables previously described and r, represents the minimum spacingbetween the passages and axis a. This centrifugal effect results in atendency of the fluid to commence rotation and produce eddy currentsjust as it enters the passage 82 in the rotor. These eddy currents, asshown in FIG. 6, seriously impede the flow of fluid into the cylinderand are frequently accompanied by cavitation which results in wear ofthe passage walls. Therefore A P must also be large enough to overcome Pas well as P and P as previously described.

Referring now to FIG. 6A, a plot is shown of pressure (psia) v. theaverage radial distance (in.) between the passages 82 and the centerlineor rotational axis a of the rotor. Where the supply of hydraulic fluidis at atmospheric pressure, e.g. 14.7 psia, that pressure is the maximumpressure which is available to supply the pump and is represented by P Pwill be diminished by P which is substantially constant for a given flowrate Q and passage area A. This P may be for example on the order of 5psia, and thus, where the radial spacing of the ports is 1.48 in. fromthe rotor centerline, the A P RESERVE still available to supply thecylinders over resistance P is 9.7 psia. As the distance of the passage82 to the rotor axis is further increased, the A P RESERVE will still befurther diminished by the increasing centrifugal effect P and tangentialeffect P these combined effects being represented by the curve P in FIG.6A. Thus, as the distance between the passages 82 and rotor axis a isincreased, it will be seen that the AP RESERVE as shown by the hatchedarea, will progressively decrease due to P which is shown superimposedon P in FIG. 6A. In fact, if the distance is increased enough, P plusP,, will intersect P and no reserve will remain, starving the By axiallyoffsetting the passages 82 toward the rotor axis from the respectiveaxes of the cylinders with which they communicate, this loss of A PRESERVE due to P is substantially reduced for a given rotor speed sincer and r, are decreased. The passages 82, rather than communicatingaxially with the cylinders 44, now communicate with the side of thecylinders nearest the rotor axis and the major portion of the passageslie even closer to the axis. This not only decreases P and P but alsotends to assist flow of the fluid present in passages 82 into thecylinders by a centrifuge effect. Thus, a substantial increase in the AP RESERVE is realized and it will be seen that the size of the pump maybe increased substantially so long as passages 82 remain relativelyclose to the rotor axis.

Referring again to FIG. 1, it will be recognized that during operationof the pump, when a given piston is to be moved outward in its cylinderon the suction stroke, the piston and its shoe will tend to be drawnaway from the tilt plate 50 due to viscous drag and inertia. For examplein FIG. 1, as the upper piston is being drawn outward, a force will beexerted on its shoe toward the right.

In order to compensate for this force and to avoid the need to lightenthe pistons by boring and/or light weight inserts in the pistons asfrequently found in conventional pistons, the piston and its shoe areurged into firm contact at all times against the surface 54 of the tiltplate by hydrostatic pressure. Each of the pistons 48 is stepped in twodiameters, a larger diameter 88 and a smaller diameter 89, the steppedannular surface at the transition between the two piston diametersproviding a piston surface 90. Corresponding portions of the cylindersleeves 46, or of the cylinder walls where sleeves are not provided, arealso stepped to provide a smaller diameter portion 93 and a largerdiameter portion 92 with a stepped surface 94 at the transition betweenthe two portions. Surface 94 is positioned in the cylinder such thatwhen its piston is fully extended into the cylinder, the surface of thepiston will still be slightly spaced from the surface 94 of thecylinder. Thus a small annular chamber 96 of varying volume is providedbetween each of the pistons 48 and their cylinder sleeves 46.

Referring now to FIGS. 1-5, pressure passage 98 is bored in the endcover and communicates with the discharge passage 86 at one end and withan axially extending passage 99 at the other end. Passage 99 extendsaxially through the center of the port plate 19 and partially into therotor 30. A plurality of radially extending passages 100 eachcommunicate passage 99 with one of the stepped piston chambers 96. Theshort remaining portions of passages 100 between the chambers and thehousing which result from the manufacturing operation, are plugged bythreaded plugs 102.

Thus each of the piston chambers 96 will be maintained at dischargepressure by way of passages 98, 99 and 100. This pressure will act uponthe annular piston surfaces 90 of the pistons to urge the pistons,particularly during the suction stroke, where the pistons tend to liftfrom the tilt plate, to the left to firmly seat their shoes against thetilt plate 50. It will be seen that the chambers 96 are effectivelyinterconnected with each other by way of passages 100. Due to thisinterconnection, little, if any, flow will occur through passages 98 and99 since fluid is simply transferred from the chamber of a given pistonwhich is on a discharge stroke to one on a suction stroke by way ofpassages 100. Passages 98 and 99 do, however, insure that the pressurein the chambers is maintained at the elevated discharge pressure.

Where the passages 82 are offset toward the rotor axis as previouslydescribed, a force couple will be generated on the rotor which tends totilt the rotor about an axis perpendicular to its rotational axis, orfor example, in a counterclockwise direction as viewed in FIG. 1 and asindicated by the arrow C. This couple is caused, by the fact the pistonswhich are on the discharge stroke will exert a force F 1 on the ends ofthe cylinders and the high pressure discharge fluid will exert a force Fon the rotor in a direction opposite the F,. If the passages 82 extendedaxially from the cylinders, F and F would balance each other. However,since the passages are offset from the cylinder axes, F and F will bespaced from each other by the offset distance d creating couple C whichtends to tilt the rotor. The tendency of the rotor to tilt results inwear to its end face 32 and face 34 of the port plate 19 which are insliding contact with each other and may result in wear at bearings 35.The couple C is substantially age gravated where the pump is of largesize. This tilting couple is cancelled by way of the stepped pistonconstruction and balance pad construction which will hereafter bedescribed.

Discussing first the stepped piston construction, it will be seen thatas pressure fluid is admitted to chambers 96, the fluid will exert aforce on shoulders 94 of the sleeves 46. These forces will combine toproduce a resultant force F which acts along the rotor axis as shown inFIG. 1.

In addition to the force F exerted by the stepped pistons, a balancingarrangement, as shown in FIGS. l-4, is also preferably provided. Thebalancing arrangement includes one or more bore passages 106 in endcover 10 which communicate with the discharge passage 86 at one end andwith a drilled passage 108 which extends through the thickness of theport plate 19 in that are of the port plate which is on the dischargeside of the pump. The passages 108 in the port plate communicate withelongated slots 110 in lands 112 on the side of the port plate facingthe rotor 30. End face 32 of the rotor slideably bears against lands 112and fluid, at discharge pressure, is thereby distributed in the .slotsto act against the rotor face and exert a force F, on the rotor. ForcesF and F are separated by distance d to produce a couple which opposesand cancels couple C.

To provide lubrication for the stationary face 34 of the port plate 19which is in sliding contact with the end face 32 of the rotor 30,suitable fluid chambers are provided as shown in FIGS. l-4. A pluralityof grooves may be formed in the inner face 20 of the cover 10, and mayinclude, for example, a circular groove 1 13 which surrounds the opening108 in the port plate 19. Groove 113 may communicate with anothercircular groove 1 14 which surrounds ports 26, 27, 28 and 29 by way ofdrain grooves 116 in the end cover 10. Thus, a small amount of the highpressure discharge fluid is leaked from passage 106 to the circulargroove 113, groove 116 and circular groove 114. In addition, thecircular groove 1 14 may also communicate with an inner circular groove118 by way of radially extending grooves 120. The grooves 114 and 120communicate by way of a plurality of drilled passages 121 and 122, withlarger draining recesses 124 and 125 respectively in the face 34 of theport plate which faces the rotating end face 32 of the rotor as shown inFIG. 4. These recesses also communicate by way of grooves 126 betweenthe bearing lands 128 of the port plate to the housing casing.

Referring again to FIG. 1, the control pistons 72, 74 and 76 arepositioned in end cover 11 to act against the side of the tilt plate 50opposite the pistons 48. This placement of the control pistons effects asubstantial reduction in the diameter of the overall pump and tilt plateand also acts to counterbalance the forces exerted on the tilt plate bypistons 48. This is particularly important where the pump is already oflarge size.

In the preferred pump, a hydraulically operated offstroke control piston72 is provided which acts against the top side of the tilt plate 50 asshown in FIG. 1. The off-stroke piston exerts a pressure on the tiltplate preferably by way of a fluid loaded ball 133 which rollablycontacts the tilt plate. The ball enables the piston to be substantiallyshortened. In addition, hydraulically operated on-stroke control piston74 is also provided which acts against the bottom side of the tilt platetending to increase the tilt of the plate and the stroke of the pistons.A starting on-stroke piston 76 is also preferably provided which actsagainst the bottom of the tilt plate. This piston 76 is mechanicallyurged against the tilt plate by the force exerted by spring 130 on aplunger member 132 whereas pistons 72 and 74 are preferablyhydraulically urged against the plate. The purpose of the startingpiston 76 is to tilt the plate to full stroke upon the starting of thepump when sufficient hydraulic pressure has not yet been generated bythe pump to actuate control pistons 72 and 74.

Referring now to FIGS. 2 and 8, a passage 134 is provided in the portcover 10 which communicates at one end with the discharge passage 18, asshown in FIG. 2, and at the other end is adapted to receive an elbowfitting 136 which is threaded into the passage. A high pressure conduit138 extends between the elbow fitting 136 and a bored passage 140 in adischarge compensator block 142 which is stationarily bolted upon endcover 1 1.- v

The compensator block 142 includes a cylindrical extension 144 having aspring 146 positioned therein which acts between threaded adjustmentplug 148 fitted in one end of the cylinder and a movable disc 150adjacent the other end of the cylinder. A spool valve 152 bears againstthe disc 150 and is positioned in a longitudinally bored passage 154 inthe block. The spool valve 152 includes an enlarged head 156 which ismovable from the left hand position shown in FIG. 8 to any one ofseveral positions to the right, by the force exerted by the pressurefluid in passage 154 which is communicated to head 156 of the spoolvalve by way of passage 158 which continuously communicates with passage140. It will be appreciated that the degree of movement of the spoolvalve 152 for a given pressure obtaining in passage 154 will depend uponthe force opposing such movement exerted by the spring 146 and thisforce is adjustable by varying the setting of the adjustment plug 148.

Referring to FIGS. 7 and 8, another passage 160 is also bored in the endcover 11 and communicates continuously between passage 140 and on-strokepiston 74 as well as the bearing 52 of the tilt plate as will beexplained in more detail later. In addition, passages 162 and 164 arealso bored in the compensator block 142 and end cover 11 and communicatethe spool valve passage 154 respectively with the off-stroke piston 72and the casing, the latter passage 164 acting as a relief passage torelieve the off-stroke piston 72 to the pump casing if the pressure inthe discharge port 18 and passage 140 drops. Suitable sealing rings 165may be provided as necessary at the junction between block 142 and cover11 to prevent leakage of the high pressure fluid.

Prior to describing the operation of the compensator, the trunnionbearing construction 52 will first be described since the bearing actsto a certain extent in conjunction with the control pistons.

Referring now to FIGS. 1, 9 and 9A, the tilt plate bearing 52 includes atrunnion block having a concave surface 172 facing the tilt plate and anarcuate pair of end walls 174 formed at each end of the concave surface.Each of the end walls 174 is convex on its outer surface 175 and concaveon the edge 176 which faces toward the tilt plate. A plurality of holes178 extend through the thickness of the end walls 174 and the trunnionblock is stationarily mounted against a step 180 in end cover 11 byscrews 181 extending through the holes 178 as shown in FIG. 1.

As shown particularly in FIG. 7, a pair of bored passages 182 and 182'extend through the end cover 1 1 and one end of each of the passagescommunicates continuously with passage 160 just before the on-strokepiston 74. The other end of passages 182 and 182' terminates adjacentthe back face 184 of the trunnion block 170 as shown in FIG. 1. Each ofthese passages 182 and 182', in tum, communicates with small boredpassages 186 and 186 respectively which extend through the thickness ofthe trunnion block and open to the concave surface 172 of the trunnionblock. Passages 182 and 182 may be slightly countersunk adjacent theback face 184 of the trunnion block so as to receive a suitable sealingring 187 which is pressed between the back face of the trunnion blockand the end cover 11 when the trunnion block has been positioned in thelater on step 180.

A pair of arcuate end pieces 188 are also positioned at each side of thetrunnion block 170. Several holes 189 extend through the width of theend pieces to accornmodate screws (not shown) for stationarily attachingthe end pieces also to the end cover 11. The concave inner surface 190of each of the end pieces is of a curvature to fit snuggly againstsurfaces 175 of the trunnion block and are formed with an arcuate groove192 for receiving the trunnions of the tilt plate.

An arcuate insert 194, having a length substantially equal to thedistance between the end walls 174 of the trunnion block, is adapted tobe received against surface 172. As shown in FIG. 9A, the side of theinsert facing the trunnion block 170, is suitably grooved at 195 and thegrooves register with passages 186 and 186.

The back surfaceof the tilt plate 50 also includes a convex surface 196which is adapted to mate with the concave face 197 of the insert 194. Apair of arcuate trunnions 198 extend from each end of the convex surface196 and are adapted to be movably inserted in the arcuate grooves 192 inthe end pieces 188 to provide for tilting of the tilt plate. Thetrunnion block 170, insert 194 and tilt plate 50 are suitably aperturedat 199 to allow for passage therethrough of the drive shaft 36 as shownin FIG. 1.

In the assembled tilt plate bearing, the convex surface 196 of the tiltplate and the concave face 197 of the insert 194 are positioned togetherand an elongated key (not shown) is inserted from the side into themating slots 200 in each surface to prevent movement of the insert andtilt plate relative to each other about the radius of curvature of theirsurfaces 196 and 197. The insert and tilt plate are then positioned inthe trunnion block 170 so as to fit between the end walls 174 of thetrunnion block. The end pieces 188 are positioned in place with thetrunnions 198 extending into their arcuate grooves 192.

During operation of the pump, grooved face 195 of the insert 194 whichis adjacent the concave surface 172 of the trunnion block, will bepressurized at all times by fluid at discharge pressure by way ofpassages 160, 182, 182' and 186, 186'. Thus, a pressurized hydraulicfluid cushion is provided between these surfaces and a force is exertedon the insert and tilt plate to counterbalance the force exerted in theopposite direction on the tilt plate by the pistons 48. This fluidcushion not only relieves stresses on the tilt plate and bearingassembly, but also acts to insulate the pump against the transmission ofsound and thereby effects a substantial reduction in pump noise levelduring operation.

The control pistons 72, 74 and 76, as well as the trunnion bearing 52,are sized such that their combined forces which act against the leftside of the tilt plate are approximately equal to the forces exertedupon the other side of the-tilt plate by the pistons 48 during operationof the pump. Thus, these latter forces which occur in all axial pistonspumps are effectively counterbalanced.

The operation of the above described system is as follows. Theadjustment plug 148 is screwed into the cylinder 144 by a predeterminedamount and thereby sets a predetermined force on the spring 146depending upon the discharge pressure desired at port 18 of the pump.The pump is then turned on. Upon starting, the hydraulic pressure neededto operate the on-stroke and off-stroke pistons 74 and 72 is not yetavailable. However, the mechanical spring urged starting piston 76 willtilt the tilt plate to its fill tilt-full stroke position. As 5 pressurebuilds up in the discharge port 18, some of the fluid at dischargepressure will pass through passage 134, fitting 136, conduit 138, andpassage 140 and will flow through the continuously open passage 160 tothe on-stroke piston 74 and tilt plate trunnion bearing.

Thereby the on-stroke piston 74 will now take over primary control ofthe tilt plate from the starting piston 76 Fluid at discharge pressurewill also be ported from passage 140, through passage 158 to passage 154and will exert a force on the left side of the enlarged head 156 ofspool valve 152. As the pressure builds up, the fluid urges the spoolvalve to the right, as viewed in FIG. 8 against the force of spring 146.When the discharge pressure reaches the desired magnitude, the spoolvalve head 156 will move past the left edge of passage 162 to allowpressurized fluid to flow to the offstroke piston 72, actuating thepiston and removing some of the tilt from the tilt plate. Thereby, thestroke of the pistons 48 will be slightly decreased to decrease the flowin port 18. The spool valve will thus move back and forth to maintainthe desired discharge pressure of the pump as set by the adjustment plug148.

If for some reason the discharge pressure in port 18 exceeds a maximumlimit, the off-stroke piston 72 is sized so as to exert a force on thetop of the tilt plate which is sufficient to cause the trunnion insert194 to separate from its block 170 by an amount sufficient to relievesome of the pressure to the pump casing from passages 182 through thebearing. Referring to FIG. 1, suitable fittings 166 are preferablyprovided for receiving a conduit which communicates with the supply orother reservoir of hydraulic fluid for passage of excess fluid from thehousing.

Conversely, if the pressure in port 18 drops, the spool valve 152 willmove to the left to the position shown in FIG. 8. In this position,passages 162 and 164 are communicated with each other and the fluid inthe off-stroke piston 72 is relieved to the pump casing by way ofpassages 162, 154 and 164, respectively to cause further tilt of thetilt plate and increase the flow through port 18.

It will be noted that the rotor of the above described pump, may becompletely surrounded by a fluid film so as to sound insulate thisrotating member to substantially reduce the operating noise level of thepump. Such fluid film is dynamically formed at the bearing 35. A film isalso formed between surfaces 32 and 34 and between the trunnion block170 and insert 194.

It will be understood that, although the various features of the fluidpressure device have been described with reference to a hydraulic pump,the features of the invention will be equally applicable to hydraulicmotors. It will also be understood that the embodiment of the inventionwhich has been described is merely illustrative of a few of theapplications of the principles of the invention. Numerous modificationsmay be made by those skilled in the art without departing from the truespirit and scope of the invention.

What is claimed is:

1. In a fluid pumping device, a high pressure discharge port and a lowpressure suction port,

a fluid source for supplying said suction port with a rotor mounted forrotation about a rotational axis,

a plurality of cylinders in said rotor alternately communicating withsaid ports when said rotor is rotated, the longitudinal axes of saidcylinders being radially spaced from the rotational axis of said rotor,each of said cylinders having a piston reciprocally movable therein,

a tilt plate drivingly associated with an end of each of said pistons,said tilt plate being angularly disposed relative to said rotationalaxis whereby when said rotor is rotated, said pistons reciprocate alongthe axes of said cylinders,

means for reducing resistance to the flow of fluid into said cylinderswhen said cylinders communicate with said suction port, said meanscomprising passage means extending between and communicating said portsand said cylinders, said passage means being radially positioned betweenthe rotational axis of said rotor and the axes of said cylinders andsubstantially near said rotational axis, whereby the fluid resistance insaid passage means resulting from rotation of said rotor issubstantially less than the pressure of the fluid supplied from saidsource,

first high pressure passage means continuously communicating a surfacein each said cylinder in said rotor with the fluid in said dischargeport, the high pressure discharge fluid continuously exerting a force onsaid surface in each said cylinder for counterbalancing said rotor, and

bearing means, said tilt plate being pivotally mounted on said bearingmeans, and high pressure fluid passage means communicating said bearingmeans with a source of high pressure fluid for exerting a force on saidtilt plate in a direction opposite to the force exerted by said pistonson said tilt plate.

2. In the pump of claim 1 wherein the distance of said first mentionedpassage means to said rotational axis and the cross sectional area ofsaid passage are of a given magnitude, whereby the resistance to fluidflow which is caused by said distance and area is substantially lessthan the pressure of said source.

3. In the pmnp of claim 1 including second high pressure passage meanscommunicating said high pressure discharge port with said rotor adjacentthe cylinders which are in communication with said discharge port,whereby fluid, substantially at discharge pressure, exerts a secondforce on said rotor which together with said first mentioned forceprovides a couple which counterbalances said rotor.

4. In the pump of claim 1 wherein said first high pressure passage meanscomprises an axially extending passage communicating with said highpressure discharge port and extending axially of said rotor, and aplurality of radially extending passages in said rotor extending betweensaid axially extending passage and each of said cylinders, said radiallyextending passages being in continuous communication with said axiallyextending passage and said axially extending passage being in continuouscommunication with said high pressure discharge port, respectively,during operation of said device.

5. In the pump of claim 1 wherein said last mentioned passage meanscommunicates said bearing means with said high pressure discharge port.

6. In the pump of claim 1 wherein said bearing means comprise firstblock means stationarily mounted in the device and having an arcuatesurface facing said tilt plate, a member positioned on and movable withsaid tilt plate and having an arcuate surface complimenting the arcuatesurface of said block means, and wherein said last mentioned passagemeans communicates the high pressure discharge port with thearcuatesurface of said block means, whereby high pressure dischargefluid is continuously introduced between said arcuate surfaces andexerts a force on the arcuate surface of said member toward said tiltplate during operation of said device.

7. In the pump of claim 3 wherein said second high pressure passagemeans communicate with said rotor between the cylinder axes and itsperimeter.

8. In the pump of claim 3 wherein said second high pressure passagemeans exerts a force on said rotor over an arcuate area.

9. In a fluid pumping device,

a high pressure discharge port and a low pressure suction port, a rotormounted for rotation about a rotational axis, a plurality of cylindersin said rotor alternately communicating with said ports when said rotoris rotated, the longitudinal axes of said cylinders being spacedradially from the rotational axis of said rotor,

a surface formed in each of said cylinders,

a plurality of pistons each reciprocally positioned in said cylinders,

an angularly disposed tilt plate drivingly associated with an end ofeach of said pistons to reciprocate said pistons in said cylinders,

, first passage means extending between and communicating said ports andsaid cylinders, said passage means being radially positioned betweensaid rotational axis and the axes of said cylinders'and substantiallynear said rotational axis,

second passage means continuously communicating said high pressuredischarge port with said rotor adjacent the cylinders which are incommunication with said discharge port,

third passage means also continuously communicating said high pressuredischarge port with said surfaces in the cylinders,

said second and third passage means continuously delivering fluid,substantially at discharge pressure during the operation of said deviceto said rotor to continuously exert forces on said rotor whichcounterbalance said rotor, and

bearing means, said tilt plate being pivotally mounted on said bearingmeans, and high pressure fluid passage means communicating said bearingmeans with a source of high pressure fluid for exerting a force on saidtilt plate in a direction opposite to the force exerted by said pistonson said tilt plate. 10. In the pump of claim 9 wherein said lastmentioned passage means communicates said bearing means with said highpressure discharge port.

11. In the pump of claim 9 wherein said bearing means comprise firstblock means stationarily mounted in the device and having an arcuatesurface facing said tilt plate, a member positioned on and movable withsaid tilt plate and having an arcuate surface complimenting the arcuatesurface of said block means, and wherein said last mentioned passagemeans communicates the high pressure discharge port with the arcuatesurface of said block means, whereby fluid at high pressure isintroduced between said arcuate surfaces and exerts a force on thearcuate surface of said member toward said tilt plate during operationof said device.

12. In a fluid pumping device, a high pressure discharge port and a lowpressure suction port,

a fluid source for supplying said suction port with a rotor mounted forrotation about a rotational axis,

a plurality of cylinders in said rotor alternately communicating withsaid ports when said rotor is rotated, the longitudinal axes of saidcylinders being radially spaced from the rotational axis of said rotor,each of said cylinders having a piston reciprocally movable therein,

a tilt plate drivingly associated with an end of each of said pistons,said tilt plate being angularly disposed relative to said rotationalaxis'whereby when said rotor is rotated, said pistons reciprocate alongthe axes of said cylinders,

means for reducing resistance to the flow of fluid into said cylinderswhen said cylinders communicate with said suction port, said meanscomprising passage means extending between and communicating said portsand said cylinders, said passage means being radially positioned betweenthe rotational axis of said rotor and the axes of said cylinders andsubstantially near said rotational axis, whereby the fluid resistance insaid passage means resulting from rotation of said rotor issubstantially less than the pressure of the fluid supplied from saidsource,

first high pressure passage means continuously communicating a surfacein each said cylinder in said rotor with the fluid in said dischargeport, the high pressure discharge fluid continuously exerting a force onsaid surface in each said cylinder for counterbalancing said rotor, and

control means for selectively exerting a force on said tilt plate topivot said tilt plate about a pivot axis to vary the stroke of saidpistons, said control means being positioned on the side of said tiltplate opposite said pistons and exerting said force on said tilt platein a direction opposite the force exerted by said pistons on said tiltplate.

13. In the pump of claim 12 including passage means communicating saidcontrol means with the high pressure fluid in said discharge port.

14. In the pump of claim 12 including bearing means pivotally mountingsaid tilt plate for rotation about said pivot axis, and meanscontinuously communicating said bearing means with said high pressuredischarge port for continuously exerting a force on said tilt plate in adirection opposite to the force exerted by said pistons on said tiltplate.

15. In a fluid pumping device,

a high pressure discharge port and a low pressure suction port,

a rotor mounted for rotation about a rotational axis,

a plurality of cylinders in said rotor alternately comnicating saidports and said cylinders, said passage means being radially positionedbetween said rotational axis and the axes of said cylinders andsubstantially near said rotational axis,

second passage means continuously communicating said high pressuredischarge port with said rotor adjacent the cylinders which are incommunication with said discharge port, third passage means alsocontinuously communicating said high pressure discharge port with saidsurfaces in the cylinders,

said second and third passage means continuously delivering fluid,substantially at discharge pressure, during the operation of said-deviceto said rotor to continuously exert forces on said rotor whichcounterbalance said rotor, and

control means for selectively exerting a force on said tilt plate topivot said tilt plate about a pivot axis to vary the stroke of saidpistons, said control means being positioned on the side of said tiltplate opposite said pistons and exerting said force on said tilt platein a direction opposite the force exerted by said pistons on said tiltplate.

16. In the pump of claim including passage means communicating saidcontrol means with the high pressure fluid in said discharge port.

17. In the pump of claim 15 including bearing means pivotally mountingsaid tilt plate for rotation about said rotated, the longitudinal axesof said cylinders being spaced radially from the rotational axis of saidrotor, a surface formed in each of said cylinders, a plurality ofpistons each reciprocally positioned in said cylinders, an angularlydisposed tilt plate drivingly associated with an end of each of saidpistons to reciprocate said pistons in said cylinders, first passagemeans extending between and communicating'said ports and said cylinders,said passage means being radially positioned between said rotationalaxis and the axes of said cylinders and substan iall near saidrotational a igs, secon ge means contlnuo y communicating said highpressure discharge port with a portion of said rotor adjacent thecylinders which are in communication with said discharge port,

third passage means also continuously communicating said high pressuredischarge port with said surfaces in thecylinders,

said second and third passage means continuously delivering fluid,substantially at discharge pressure during the operation of said deviceto said rotor to continuously exert forces on said rotor, and

said surfaces in said cylinders and said portion of said rotor beingdimensioned and being spaced from the rotational axis of said rotor suchthat the force exerted upon said rotor by the fluid which communicateswith said surfaces and the force exerted upon said rotor by the fluidwhich communicates with said portion of said rotor define acounterbalancing mechanical couple which substantially counterbalancesany unbalancing forces exerted upon said rotor by said radiallypositioned first passage means.

19. In the pump of claim 18 wherein said first passage means communicatethrough the side of said cylinders adjacent said rotational axis.

20. In the pump of claim 18 wherein said third passage means comprisesan axially extending passage communicating with said high pressuredischarge port and extending axially of said rotor, and a plurality ofradially extending passages in said rotor extending betweensaid axiallyextending passage and each of said cylinders, said radially extendingpassages being in continuous communication with said axially extendingpassage and said axially extending passage being in continuouscommunication with said high pressure 0 discharge port, respectively,during operation of said device.

P0405!) UNITED STA S PATENT @WICE csTl slcATs s mews PatentNo 3, 682,044 Dated August 8, 1972 Jay E. Arikeny, Harold W. Foddy and Dayton F.Kessler Inventor(s) It is certifiedthat error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

On the first page 'of the patent, in the first column under 7 "ReferenceCited:

2, 232,984 2/1941 Wahlmark 74/60 3,384,028 5/1968 Thoma 91/501 areadded. Column 3, line 33, "section" should read suction Signed andsealed this 22nd day of May 1973.

(SEAL) f Attest:

I TCHER JR. ROBERT GOTTSCHALK EDWARD M FLE' I Commissioner of PatentsAttesting Officer

1. In a fluid pumping device, a high pressure discharge port and a lowpressure suction port, a fluid source for supplying said suction portwith fluid, a rotor mounted for rotation about a rotational axis, aplurality of cylinders in said rotor alternately communicating with saidports when said rotor is rotated, the longitudinal axes of saidcylinders being radially spaced from the rotational axis of said rotor,each of said cylinders having a piston reciprocally movable therein, atilt plate drivingly associated with an end of each of said pistons,said tilt plate being angularly disposed relative tO said rotationalaxis whereby when said rotor is rotated, said pistons reciprocate alongthe axes of said cylinders, means for reducing resistance to the flow offluid into said cylinders when said cylinders communicate with saidsuction port, said means comprising passage means extending between andcommunicating said ports and said cylinders, said passage means beingradially positioned between the rotational axis of said rotor and theaxes of said cylinders and substantially near said rotational axis,whereby the fluid resistance in said passage means resulting fromrotation of said rotor is substantially less than the pressure of thefluid supplied from said source, first high pressure passage meanscontinuously communicating a surface in each said cylinder in said rotorwith the fluid in said discharge port, the high pressure discharge fluidcontinuously exerting a force on said surface in each said cylinder forcounterbalancing said rotor, and bearing means, said tilt plate beingpivotally mounted on said bearing means, and high pressure fluid passagemeans communicating said bearing means with a source of high pressurefluid for exerting a force on said tilt plate in a direction opposite tothe force exerted by said pistons on said tilt plate.
 2. In the pump ofclaim 1 wherein the distance of said first mentioned passage means tosaid rotational axis and the cross sectional area of said passage are ofa given magnitude, whereby the resistance to fluid flow which is causedby said distance and area is substantially less than the pressure ofsaid source.
 3. In the pump of claim 1 including second high pressurepassage means communicating said high pressure discharge port with saidrotor adjacent the cylinders which are in communication with saiddischarge port, whereby fluid, substantially at discharge pressure,exerts a second force on said rotor which together with said firstmentioned force provides a couple which counterbalances said rotor. 4.In the pump of claim 1 wherein said first high pressure passage meanscomprises an axially extending passage communicating with said highpressure discharge port and extending axially of said rotor, and aplurality of radially extending passages in said rotor extending betweensaid axially extending passage and each of said cylinders, said radiallyextending passages being in continuous communication with said axiallyextending passage and said axially extending passage being in continuouscommunication with said high pressure discharge port, respectively,during operation of said device.
 5. In the pump of claim 1 wherein saidlast mentioned passage means communicates said bearing means with saidhigh pressure discharge port.
 6. In the pump of claim 1 wherein saidbearing means comprise first block means stationarily mounted in thedevice and having an arcuate surface facing said tilt plate, a memberpositioned on and movable with said tilt plate and having an arcuatesurface complimenting the arcuate surface of said block means, andwherein said last mentioned passage means communicates the high pressuredischarge port with the arcuate surface of said block means, wherebyhigh pressure discharge fluid is continuously introduced between saidarcuate surfaces and exerts a force on the arcuate surface of saidmember toward said tilt plate during operation of said device.
 7. In thepump of claim 3 wherein said second high pressure passage meanscommunicate with said rotor between the cylinder axes and its perimeter.8. In the pump of claim 3 wherein said second high pressure passagemeans exerts a force on said rotor over an arcuate area.
 9. In a fluidpumping device, a high pressure discharge port and a low pressuresuction port, a rotor mounted for rotation about a rotational axis, aplurality of cylinders in said rotor alternately communicating with saidports when said rotor is rotated, the longitudinal axes of saidcylinders being spaced radially from the rotational axis of saId rotor,a surface formed in each of said cylinders, a plurality of pistons eachreciprocally positioned in said cylinders, an angularly disposed tiltplate drivingly associated with an end of each of said pistons toreciprocate said pistons in said cylinders, first passage meansextending between and communicating said ports and said cylinders, saidpassage means being radially positioned between said rotational axis andthe axes of said cylinders and substantially near said rotational axis,second passage means continuously communicating said high pressuredischarge port with said rotor adjacent the cylinders which are incommunication with said discharge port, third passage means alsocontinuously communicating said high pressure discharge port with saidsurfaces in the cylinders, said second and third passage meanscontinuously delivering fluid, substantially at discharge pressureduring the operation of said device to said rotor to continuously exertforces on said rotor which counterbalance said rotor, and bearing means,said tilt plate being pivotally mounted on said bearing means, and highpressure fluid passage means communicating said bearing means with asource of high pressure fluid for exerting a force on said tilt plate ina direction opposite to the force exerted by said pistons on said tiltplate.
 10. In the pump of claim 9 wherein said last mentioned passagemeans communicates said bearing means with said high pressure dischargeport.
 11. In the pump of claim 9 wherein said bearing means comprisefirst block means stationarily mounted in the device and having anarcuate surface facing said tilt plate, a member positioned on andmovable with said tilt plate and having an arcuate surface complimentingthe arcuate surface of said block means, and wherein said last mentionedpassage means communicates the high pressure discharge port with thearcuate surface of said block means, whereby fluid at high pressure isintroduced between said arcuate surfaces and exerts a force on thearcuate surface of said member toward said tilt plate during operationof said device.
 12. In a fluid pumping device, a high pressure dischargeport and a low pressure suction port, a fluid source for supplying saidsuction port with fluid, a rotor mounted for rotation about a rotationalaxis, a plurality of cylinders in said rotor alternately communicatingwith said ports when said rotor is rotated, the longitudinal axes ofsaid cylinders being radially spaced from the rotational axis of saidrotor, each of said cylinders having a piston reciprocally movabletherein, a tilt plate drivingly associated with an end of each of saidpistons, said tilt plate being angularly disposed relative to saidrotational axis whereby when said rotor is rotated, said pistonsreciprocate along the axes of said cylinders, means for reducingresistance to the flow of fluid into said cylinders when said cylinderscommunicate with said suction port, said means comprising passage meansextending between and communicating said ports and said cylinders, saidpassage means being radially positioned between the rotational axis ofsaid rotor and the axes of said cylinders and substantially near saidrotational axis, whereby the fluid resistance in said passage meansresulting from rotation of said rotor is substantially less than thepressure of the fluid supplied from said source, first high pressurepassage means continuously communicating a surface in each said cylinderin said rotor with the fluid in said discharge port, the high pressuredischarge fluid continuously exerting a force on said surface in eachsaid cylinder for counterbalancing said rotor, and control means forselectively exerting a force on said tilt plate to pivot said tilt plateabout a pivot axis to vary the stroke of said pistons, said controlmeans being positioned on the side of said tilt plate opposite saidpistons and exerting said force on said tilt platE in a directionopposite the force exerted by said pistons on said tilt plate.
 13. Inthe pump of claim 12 including passage means communicating said controlmeans with the high pressure fluid in said discharge port.
 14. In thepump of claim 12 including bearing means pivotally mounting said tiltplate for rotation about said pivot axis, and means continuouslycommunicating said bearing means with said high pressure discharge portfor continuously exerting a force on said tilt plate in a directionopposite to the force exerted by said pistons on said tilt plate.
 15. Ina fluid pumping device, a high pressure discharge port and a lowpressure suction port, a rotor mounted for rotation about a rotationalaxis, a plurality of cylinders in said rotor alternately communicatingwith said ports when said rotor is rotated, the longitudinal axes ofsaid cylinders being spaced radially from the rotational axis of saidrotor, a surface formed in each of said cylinders, a plurality ofpistons each reciprocally positioned in said cylinders, an angularlydisposed tilt plate drivingly associated with an end of each of saidpistons to reciprocate said pistons in said cylinders, first passagemeans extending between and communicating said ports and said cylinders,said passage means being radially positioned between said rotationalaxis and the axes of said cylinders and substantially near saidrotational axis, second passage means continuously communicating saidhigh pressure discharge port with said rotor adjacent the cylinderswhich are in communication with said discharge port, third passage meansalso continuously communicating said high pressure discharge port withsaid surfaces in the cylinders, said second and third passage meanscontinuously delivering fluid, substantially at discharge pressure,during the operation of said device to said rotor to continuously exertforces on said rotor which counterbalance said rotor, and control meansfor selectively exerting a force on said tilt plate to pivot said tiltplate about a pivot axis to vary the stroke of said pistons, saidcontrol means being positioned on the side of said tilt plate oppositesaid pistons and exerting said force on said tilt plate in a directionopposite the force exerted by said pistons on said tilt plate.
 16. Inthe pump of claim 15 including passage means communicating said controlmeans with the high pressure fluid in said discharge port.
 17. In thepump of claim 15 including bearing means pivotally mounting said tiltplate for rotation about said pivot axis, and means continuouslycommunicating said bearing means with said high pressure discharge portfor continuously exerting a force on said tilt plate in a directionopposite to the force exerted by said pistons on said tilt plate.
 18. Ina fluid pumping device, a high pressure discharge port and a lowpressure suction port, a rotor mounted for rotation about a rotationalaxis, a plurality of cylinders in said rotor alternately communicatingwith said ports when said rotor is rotated, the longitudinal axes ofsaid cylinders being spaced radially from the rotational axis of saidrotor, a surface formed in each of said cylinders, a plurality ofpistons each reciprocally positioned in said cylinders, an angularlydisposed tilt plate drivingly associated with an end of each of saidpistons to reciprocate said pistons in said cylinders, first passagemeans extending between and communicating said ports and said cylinders,said passage means being radially positioned between said rotationalaxis and the axes of said cylinders and substantially near saidrotational axis, second passage means continuously communicating saidhigh pressure discharge port with a portion of said rotor adjacent thecylinders which are in communication with said discharge port, thirdpassage means also continuously communicating said high pressuredischarge port with said surfaces iN the cylinders, said second andthird passage means continuously delivering fluid, substantially atdischarge pressure during the operation of said device to said rotor tocontinuously exert forces on said rotor, and said surfaces in saidcylinders and said portion of said rotor being dimensioned and beingspaced from the rotational axis of said rotor such that the forceexerted upon said rotor by the fluid which communicates with saidsurfaces and the force exerted upon said rotor by the fluid whichcommunicates with said portion of said rotor define a counterbalancingmechanical couple which substantially counterbalances any unbalancingforces exerted upon said rotor by said radially positioned first passagemeans.
 19. In the pump of claim 18 wherein said first passage meanscommunicate through the side of said cylinders adjacent said rotationalaxis.
 20. In the pump of claim 18 wherein said third passage meanscomprises an axially extending passage communicating with said highpressure discharge port and extending axially of said rotor, and aplurality of radially extending passages in said rotor extending betweensaid axially extending passage and each of said cylinders, said radiallyextending passages being in continuous communication with said axiallyextending passage and said axially extending passage being in continuouscommunication with said high pressure discharge port, respectively,during operation of said device.